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Shark hunting and conservation

Article curated by Rowena Fletcher-Wood

Sharks are cartilaginous fish related to rays - and one of our most poorly understood animals. For example, most are picky eaters, with very specific diets that depend on their physiology, habitat and social dynamics – all of which might come into play to explain the small number of shark attacks every year.

In the wild, shark nose butting and circling is often interpreted as an intention to attack, but shark scientists think it is merely curiosity. Sharks circle because they can’t keep still and breathe. By swimming in circles with their back towards the open ocean, or a figure of eight shape, they can keep the curious thing in sight and continue to monitor and explore it with nose butting. If the shark decides the object is neither edible, interesting nor threatening, it will swim away. Sharks do go through a distinct pattern of movements before they attack, but this is not circling and butting: they first arch their back and drop their pectoral fins, then throw back their head to get into a good biting position and roll their eyes back, making them temporarily blind. As they charge, they thrash their tail rapidly. If you are under an attack by a shark, you probably don’t have time to notice all of this before they are upon you: it is certainly a far cry from circling and butting.

Attacks by sharks are actually extremely uncommon. You have a better chance of winning the lottery, and most attacks are provoked - probably a consequence of people misinterpreting circling behaviour. On average, there are 100 shark attacks worldwide every year, of which 4 are fatal[1]. Of 360 recognised shark species, just 5 attack humans unprovoked: the great white, the hammerhead, the bull, the oceanic whitetip and the tiger shark. The oceanic whitetip shark, however, lives far out to sea, and rarely comes into contact with humans.

No one is quite sure why sharks attack humans, and the reason may differ shark to shark, situation to situation. Because of the commonly misinterpreted “circling and nosebutting” behaviour, many scholars think that shark attacks may be the result of curiosity - either scaring humans into provoking an attack, or from highly confused sharks to taking an exploratory bite. This could also go part way towards explaining why so many shark attacks are not fatal.

Another common explanation is that the sharks screwed up; above them floating on the surface, they thought they saw the sleek black body of a seal - and bit it - but it didn’t taste right, and their electroreceptors picked up very different kinds of movements. Some think shark attacks would be a lot more common if our movements provided electrical signals much more like a seal's. This explanation is not very popular amongst experts, because sharks have so many well developed senses, it seems impossible they could mess up on that scale - it could even be very dangerous for them. However, it does explain why sharks will bite a human once only and then back off a bit: perhaps they are surprised by the taste, and perhaps we don’t taste good (or blubbery enough). Another possibility is that sharks are biting to test what we will do, or that they are waiting for us to die of blood loss without exhausting themselves. Interestingly, if two or more possible prey victims are present, sharks are consistently observed only attacking one: the first one they bit. No amount of shouting or splashing will deter them, probably because at close quarters sharks are driven primarily by electrical signals, which are magnified when salty bodily fluids pour out of a wounded victim. Paired with the smell of blood, the shark is experiencing a sensory overload pointing to one prey item: the others will be forgotten. This actually has the potential to make sharks vulnerable, assuming a human was in any way a match for them.

Shark attacks also relate to shark numbers. When there are more sharks around, there are more attacks. Ocean scientists think that these sharks are all parts of accessory populations: the non-mating outliers who live a bit further from the main population. Not prioritised for food for reproductive gain, they live in less lucrative hunting territories, often in shallow, coastal waters, where they are more likely to come into contact with humans. Mathematicians working on seal distribution have been able to map which size sharks live where using the same probability density maps employed to locate serial killers. Because the shark hierarchy is determined by size, it is easy to identify “desirable” spots: that’s where the big sharks are. Noticeably, the biggest sharks are not found right up against the shore where the greatest chance exists of encountering a seal; here, although seal encounters are high, if the shark charges at the seal and misses, the seal will make a leap for the shore and for safety. Further out, the seals are fewer, but if the first charge misses, the sharks get a second go. Interestingly, there exists a sweet spot in between, where seal encounters are often enough and strike rates high enough before the seal makes it to shore. This is where the big sharks are found. Surrounding the biggest sharks are a knot of medium sharks, and the smallest sharks are scattered further away, and more chaotically.

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Christian Jensen
Whale shark being touched by divers. Image credit: Public domain

It is often said that sharks are indiscriminate predators, who will literally eat anything. This is true for one shark: the tiger shark. Tiger sharks don’t seem to care what they eat, and reports of extraordinary things lifted from the stomachs of sharks usually refer to tiger sharks. Most sharks, however, are very picky about what they eat, and some are not even predators. The two largest sharks, the whale shark and the basking shark, are filter feeders, not hunters. They have the same diet as whales. When they eat, they open up their mouths to make a cylindrical tube, and swim at swarms of plankton, which they simply suck inside their massive forms. This low effort feeding technique allows the whale shark to grow up to 12.5 metres long, and means it is so uninterested in people, divers are able to ride on its back.

Almost all species of sharks are carnivorous, most sticking to one particular form of prey with minimal variations. Seals are a favourite on the shark menu, especially for older and bigger sharks. Blubbery marine mammals have a higher fat content than fish, and so a shark needs to eat fewer of them to get their fill. The truth of the matter is just that: sharks are lazy, and so they are also picky. This laziness is facilitated by the ability of a shark to store nutrients like a camel. This means it can go for a very long time without eating, if it can’t be bothered. Large sharks with big energy stores, like the great white, are actually the pickiest of them all. Younger and smaller sharks often prefer fish to marine mammals, possibly because they are smaller and easier to catch, or because their tooth shape is different. Sharks, unlike us, continuously regrow and replace their teeth. A single shark may grow 300 rows of 100 teeth during its lifetime. Some sharks are biters and slicer, with rows of sharp, triangular teeth, good for taking chunks. Others are snatchers and rippers, tearing flesh off their prey with pointier-shaped teeth. Young shark teeth tend to be pointer, and better at impaling fish than at taking chunks out of seals. Other sharks prefer to swallow their prey whole.

According to habits, scientists have identified two kinds of eaters amongst the sharks: number maximisers, that eat whatever comes their way, like the tiger shark, and energy maximisers, that prioritise high energy prey and conserve energy for hunting these. Some sharks, like the blue shark, minimise their energy expenditure hunting by cruising and diving to locate prey, whilst the mako shark alternatively swims furiously over large distances to maximise the probability of prey encounters. Many sharks, especially small ones, are pack hunters, whilst bigger sharks like the great white are predominantly but not exclusively lone hunters. However, even these patterns are subject to variation, for example, seasonally, and not all recorded data agrees on shark activity levels.

Shark digestion is a slow process. In the tail end of a J-shaped stomach, the food gets dumped ready for slow, recirculating digestion along a very short small intestine. If the shark has eaten something it shouldn't, like a surfboard, for example, it will later vomit up the indigestible items, either conventionally or by inverting its stomach through it mouth.

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Rowena Fletcher-Wood
Large sharks get the pick of seal hunting sites, and prefer a spot a bit off the shore where the chance of seals escaping to shore if they miss first time is low enough, but there are still lots of seals about. Image credit: Rowena Fletcher-Wood

Although most sharks don't eat humans, and those that do don't make a habit of it, sharks are still culled on a massive scale. More humans even eat sharks than sharks eat humans, and a major culprit is the Chinese shark fin soup trade. In addition, the long-standing pseudoscientific belief that sharks don’t get cancer has led to the consumption of shark “pills” worldwide. Unfortunately, sharks do get cancer, but they may get it less than humans (even humans who don’t smoke). We don’t know if there is a way animals can be immune to cancer, but the belief that sharks were such an animal arose on account of being cartilage rather than bone-based. Cartilage is thought to limit the growth of cancerous cells. As such, shark cancer is a little different from human cancer and involves cartilage-based tumours known as chondromas[2]. Whilst very little has been recorded on shark cancer and there is still much to be discovered about it, like the incidences and rates, the concept that sharks are cancer-resistant is not supported.

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List of Institutes Researching Rates of Cancer and Disease in Sharks
Things We Don’t Know (1 researchers)
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Around 100 million are destroyed by humans every year, equivalent to 11,000 an hour[3][4]. Compare this to around 4 human deaths by sharks every year and the logic of culling no longer makes sense (the number of human deaths per year varies statistically, from less than one to about 10). Furthermore, it could have dire consequences for ocean ecology. We know that apex predators make a huge impact on the environment in which they inhabit, not just by controlling prey population numbers, but also contributing to biodiversity through their movements, habitats, prey choice and chemistry. This is known as a trophic cascade. We have seen some of the impacts apex predators can have in their immediate environment through observations of other species: contrary to expectation, killing whales was found to reduce fish numbers not increase them, because whales maintain a habitable environment for the fish. However, the full implications of sharks on their environment is not known. Because of this unknown, ecologists are wary about the consequences of culling sharks, and fear it could massively influence the ecosystem and marine environment, including human enjoyment of the ocean for sport, seafood, and sea life. Once culled, shark numbers are difficult to recover and so impacts hard to reverse; sharks, like humans, mature slowly and have few young - taking about 15 years to reach adulthood and breeding successfully every two or three years.

Various methods are used to cull sharks, but the most well known is the controversial shark net. These are gilled nets designed to hook even the tough skins of sharks. However, they can capture, kill and damage all kinds of ocean creatures, and are considered extremely cruel and unnecessary. In New Zealand, one academic stated, The notion that we need to kill any animal that might place us at risk when we enter the water is a totally unacceptable attitude in the modern world..

Culling sharks does not act as a shark deterrent. Selective hunts to catch individual perpetrators are not found to affect the overall rate of incidents, as one highly expensive Hawaiian study eventually concluded. A closer-to-home example of why this may not work is badgers. In order to limit the spread of bovine TB, which is carried by badgers, badgers in the UK have been extensively culled despite evidence that culling badgers does not prevent the spread of bovine TB. Not only does bovine TB still spread from cow to cow in badger-free environments, but culling them is ineffective because instead of being repelled from a site where previous badger inhabitants were “disappeared”, new badgers migrate to areas with recently vacated setts, such that culling them only encourages a wider spread of disease-carrying animals.

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List of Institutes Researching The ecological role of sharks
Things We Don’t Know (1 researchers)
Sharks in light water Image credit:  

Amongst people, culling creates fear, yet doesn't solve the problem of safety in the ocean. In tourist areas in Australia it is legal to kill sharks, even protected species, if they pose an imminent threat to people. The difficulty comes in deciding what constitutes an imminent threat. After all, if bathers are removed from the water, the threat no longer exists - they just can’t go back in. People are still hopeful of developing effective shark repellents that will allow us to share the ocean with the beasts. Sensible precautions do exist that can enhance human safety without the aid of technology, such as not wearing shiny jewellery in the water, splashing excessively or diving during shark peak feeding times, in the early morning or at dusk. After a storm, fish tend to be particularly active, attracting sharks to an area and making bathing more hazardous.

At the moment, however, most shark repellent technology is being developed in a lab, and is yet to be tested on wild animals in open ocean. Some systems that are currently available include spear guns, used to prod inquisitive noses, and shark shields - portable electric field devices. These devices theoretically have a range up to 3m, and have been test proven to deter sharks. However, prolonged testing on great whites showed that although the sharks initially disliked the field, they eventually learnt to ignore it, or tune it into the background. Scientists have proposed that a changing rather than constant electric field is required to act as a permanent repellent.

Other ways people use to repel sharks are through monitoring and communications. Some sharks are tagged by researchers, and these sharks are easy to monitor. In some places, technology exists to immediately transmit warnings on social media when a shark crosses into an area. Rescue helicopters and shark watches can look out for unmarked animals, and report back to the same public outlets. These instant bulletin systems have provided the means for greatly accelerated human communications and so safety.

Some recent work on shark repellent technology has encompassed initial field tests on electrical pulses, and bubble curtains that interfere with the lateral line. An underwater barrier of bubbles oozing from pipes laid on the ocean floor, the bubble curtain makes a noise and perturbation that acts as a disturbance to sharks. Early evidence suggests they will be repelled, but it is uncertain whether, like the electric signals, they might come to adjust to it in time. Researchers are not quite certain whether the shark is scared or whether its senses are simply overwhelmed - bubble curtain research is ongoing.

Other work focuses on new visual technology. When divers buy wetsuits it is common for them to ask “What colour do sharks like?” and say, “I don’t want to look like a seal!” Since sharks are highly visual hunters, it makes sense to design wetsuits and surfboards to look as untasty as possible, or camouflage swimmers into the background. Designs have appeared in blues, greens and ripple effects. However, new evidence suggesting sharks may be colour blind - or at least some of them - has led to the development of direct repellent patterns that look like the black and white striped sea snake poisonous for sharks to eat. Sound signals have also been explored. Known as the ‘startle response’, researchers have noticed that very loud or pulsed tones creates a panic response in lemon and silky sharks, especially if it gets louder when they get closer[5][6]. It doesn’t work for oceanic whitetip sharks, however, and how, why, and when it works is yet to be pinpointed.

One elusive goal of shark repellent research is an effective and environmentally safe chemical repellent. In the USA, research has been running since 1942. So sought after is this target that many are uncertain whether such a product is scientific or merely mythological.

Researchers have identified several substances that produce effects in sharks. It is significant to note, however, the conditions under which they register a response. Since many of the test subjects are held in captivity and regularly exposed to similar situations, it is often impossible to disentangle their environmental responses from their chemical responses. As such, scientists have defined an effective “repellent” as one that will rouse a shark from tonic immobility, a restful state of paralysis they enter when turned upside down. There exists no clear argument why chemicals that rouse a shark from immobility must necessarily have a negative impact, however, and experimental results must be interpreted with caution. Under these conditions, cheap synthetic surfactants, such as sodium lauryl sulfate (the stuff that makes shampoo foam), were tested on laboratory and field sharks with some effect[7]. Environmental impacts have not been exhaustively assessed.

Natural equivalents do exist. The red sea moses sole and peacock sole release a milky mucus when bitten that repels sharks; this mucus contains a peptide called pardaxin, which breaks down mammalian and bacterial cells, causing jaw paralysis and urea secretion in the gills of a shark. Having once bitten such a sole, a shark will never try again. Pardaxin and equivalents have, however, been discounted as potential commercial chemical shark repellents because they dilute too rapidly in water and are only effective when directly introduced to the mouth of an offending shark.

Naturally occurring and detectable at much greater distances, there is one naturally occurring chemical that causes the flight of hundreds of sharks, even at the expense of a whole season's feeding. It is known as the stench of death, and it is found within the distinct chemical profile of a dead shark. Such chemicals are known as necromones. Necromones are fantastically potent, extending around a couple of square miles from the source within very short periods of time, initiating the evacuation of entire shark populations.

Sharks exist in an osmotic balance with the surrounding seawater, and as such are highly sensitive to small changes in salinity. Their body fluids are rich in urea and trimethylamine N-oxide salts, which allow them to balance internal and external sensations in a marine environment, but also make it impossible for most species to survive in freshwater. When a shark dies and their bodies decompose, a complex soup of amino acids and putrefaction products is exuded. Urea is broken down into ammonia and ammonium acetate becomes the major chemical constituent decaying shark tissue. Gas chromatography has been used to isolate some of these chemicals, but more research is needed to find the active ingredient that alerts sharks to the death of one of their own.

This 'death recognition system' may be an ancient survival tactic founded 400 million years ago, and exist across more species than we realise. Thought to be present in species more closely related to the oceans, other distinct chemical profiles have been recognised in ants, bees, termites and other insects, including oleic and linoelic acids. Noticeably, shark experiments have shown that the signals are not ubiquitous between species: sharks are not deterred by urea, oleic acid in ethanol, or buffered water, nor the sounds, bubbles nor solvents used to introduce them to water, nor do they get used to the necromones over time[8]. They simply flee.

An interesting example of this was seen off the Farallon Islands of California in 1997, in the first reported eye witness account of predation of a great white shark in the wild (other than by humans)[9]. It was October, the season that great whites come to the Farallon Islands to feast upon seals, when a pair of killer whales were spotted by whale watchers. Watching from the boat, observers saw the largest orca attack and kill a great white, following which the two whales fed upon the shark’s liver. Although dolphins have been known to kill sharks by head butting the soft region of their bellies until internal haemorrhaging occurs, this behaviour is not predatory, and dolphins will not eat the dead shark. Some believe the behaviour is protective, and although many incidents do occur when curious sharks approach dolphin calfs, dolphins are observably selective in attacking sharks that do not typically pose a threat to dolphins, preferring to flee from rather than attack those that do (such as great whites). Unlike the killer whale, dolphins attack sharks in packs, using superior numbers to overwhelm the better-equipped apex predator.

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Whilst much remains to be learnt about shark hunting and dietary behaviour, human panic has caused more accidents than shark hunger. However, with the advent of new and inventive shark deterrent technology, ocean scientists hope to give people the confidence they need to enjoy the water harmoniously with sharks, by keeping their distance and avoiding the environmental impacts of culling.


This article was written by the Things We Don’t Know editorial team, with contributions from Rowena Fletcher-Wood.

References
why don’t all references have links?

[1] International Shark Attack File Florida Museum of Natural History
[2] Finkelstein, Joel B. "Sharks do get cancer: few surprises in cartilage research." Journal of the National Cancer Institute 97.21 (2005): 1562-1563
[3] Clarke, S,C., et al., (2006) Global estimates of shark catches using trade records from commercial markets Ecology Letters 9(10):1115-1126 DOI: 10.1111/j.1461-0248.2006.00968.x
[4] Worm, B., et al., (2013) Global catches, exploitation rates, and rebuilding options for sharks Marine Policy 40:194-204 DOI: 10.1016/j.marpol.2012.12.034
[5] Banner, Arnold. "Use of sound in predation by young lemon sharks, Negaprion brevirostris (Poey)." Bulletin of Marine Science 22.2 (1972): 251-283
[6] Myrberg Jr, Arthur A. "Underwater sound-its effect on the behavior of sharks." Sensory Biology of Sharks, Skates and Rays, US Government Printing Office, Washington DC (1978): 391-417
[7] Gruber, S. H., E. Zlotkin, and D. R. Nelson. "Shark repellents: behavioral bioassays in laboratory and field." Toxins, Drugs, and Pollutants in Marine Animals. Springer Berlin Heidelberg, 1984. 26-42
[8] Chemical shark repellent: Myth or fact? The effect of a shark necromone on shark feeding behavior, Eric M. Stroud, Craig P. O’Connell, Patrick H. Rice, Nicholas H. Snow, Brian B. Barnes, Mohammed R. Elshaer, James E. Hanson, Ocean & Coastal Management 97 (2014) 50e57
[9] National Geographic Channel "The whale that ate Jaws" 11/29/2009



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